1. Field of Invention
The present invention relates to a circuit board in which respective boards having wires are connected together electrically, and to a method for jointing a circuit board.
2. Description of the Related Art
In flip chip mounting for installing an electronic component on a circuit board, bumps are formed on the wire terminals of the board and the electrodes of the electronic component. As a technique for forming bumps of wire terminals, in recent years, a method for forming bumps by self-assembly of conductive particles (for example, solder powder) on the wire terminals of a board and the electrodes of an electronic component has come to be used instead of the conventional methods of “solder pasting” and “super soldering”. Alternatively, a method has also been proposed for flip chip mounting of an electronic component on a circuit board by forming a conductor between the wire terminals of the board and the electrodes of the electronic component by self-assembly of conductive particles between the board and the electrodes of the electronic component (see, for example, Japanese Patent Publication No. 3964911 and Japanese Patent Publication No. 3955302).
FIGS. 9A to 9D and FIGS. 10A to 10D show prior art technology for forming bumps by self-assembly of conductive particles.
Firstly, as shown in FIG. 9A, a resin 114 containing solder powder 116 and a bubble generating agent (not illustrated) is supplied onto a board 31 having a plurality of pad electrodes 32.
Thereupon, as shown in FIG. 9B, a flat sheet 140 is laid on the surface of the resin 114.
When the resin 114 is heated in this state, as shown in FIG. 9C, gas bubbles 30 are produced from the bubble generating agent contained in the resin 114. Furthermore, as shown in FIG. 9D, a portion of the resin 114 is pushed out by the generated gas bubbles 30 as they grow.
As shown in FIG. 10A, the resin 114 pushed out in this way self-assembles into column shapes at the interfaces with the pad electrodes 32 of the board 31 and the interfaces with the flat sheet 140. Furthermore, a part of the resin 114 which is present in the edge portions of the board 31 is pushed out beyond the outer edges of the board 31 (not illustrated).
Thereupon, if the resin 114 is heated further, then the solder powder 116 contained in the resin 114 melts as shown in FIG. 10B, and the respective particles of solder powder 116 contained in the resin 114 which has self-assembled over the pad electrodes 32 melt and join together.
The pad electrodes 32 have higher wettability with respect to the solder powder 116 that has melted and joined together, and therefore bumps 19 made of molten solder powder are formed on the pad electrodes 32, as shown in FIG. 10C.
Finally, the resin 114 and the flat sheet 140 are removed as shown in FIG. 10D, thereby yielding a board 31 on which bumps 19 have been formed on the pad electrodes 32.
In the steps described above, the amount of resin 114 supplied is depicted in exaggerated fashion, and in actual practice, an amount of resin 114 which is suitable for self-assembly on the pad electrodes 32 and which allows for errors is supplied.
The characteristic feature of this prior art method lies in that by heating the resin 114 supplied between the board 31 and the flat sheet 140, gas bubbles 30 are produced by the bubble generating agent, the resin 114 is pushed to the exterior of the gas bubbles as the gas bubbles 30 grow, and thereby the resin 114 which still contains the solder powder 116 self-assembles between the pad electrodes 32 of the board 31 and the flat sheet 140.
The phenomenon of the resin 114 self-assembling on top of the pad electrodes 32 is thought to occur as a result of the mechanism shown in FIG. 11A and FIG. 11B.
FIG. 11A shows a state where the resin 114 has been pushed out onto the pad electrodes 32 of the board 31 by the gas bubbles as they grow (not illustrated). In the resin 114 which makes contact with the pad electrode 32, the force Fs which corresponds to the surface tension at the interface (the force generated by the wetting and spreading of the resin) is greater than the stress Fη produced as a result of the viscosity η of the resin, and therefore the resin 114 spreads over the whole surface of the pad electrode 32 and ultimately, a column-shaped resin having borders at the edges of the pad electrode 32 is formed between the pad electrode 32 and the flat sheet 140.
Although the stress Fb created by the growth (or movement) of the gas bubbles 30 acts on the column-shaped resin 114 formed by self-assembly on the pad electrodes 32 as shown in FIG. 11B, due to the action of the stress Fη created by the viscosity η of the resin 114, the shape of the resin can be maintained and hence the resin 114 which has self-assembled onto the pad electrodes 32 is never eliminated.
Here, the capability to maintain the prescribed shape of the resin 114 which has self-assembled is dependent on the surface area S of the pad electrodes 32, the interval L between the pad electrodes 32 and the flat sheet 140, and the viscosity η of the resin 114, in addition to the stress Fs corresponding to the surface tension. If the reference value for maintaining the resin 114 with a prescribed shape is taken as “T”, then a relationship of the following kind can be established in respect of the stability.T=K·(S/L)·η·Fs (K: constant)
In this way, the prior art method forms a resin 114 on the pad electrodes 32 in a self-aligning fashion by utilizing a self-assembly phenomenon resulting from the surface tension of the resin 114, and it can be seen as utilizing the fact that the self-assembly due to the surface tension occurs in between the flat sheet 140 and the pad electrodes 32 where the gap between the board 31 and the flat sheet 140 is most narrow because the pad electrodes 32 are formed in a projecting shape on the surface of the board 31.
If this prior art method is used, it is possible to make the solder powder dispersed inside the resin 114 self-assemble efficiently on top of the pad electrodes, and therefore bumps can be formed with excellent uniformity and a high rate of productivity.
Furthermore, since the solder powder which is dispersed in the resin can be made to self-assemble indivisibly onto the plurality of electrodes on the board onto which the resin has been supplied, then this method is particularly useful when forming bumps uniformly and simultaneously onto all of the electrodes of a circuit board.
The technology for self-assembling solder powder by causing self-assembly of resin as described above can be used for other applications, apart from forming bumps.
The present inventors have discovered, as one application of this kind, the use of such technology in jointing circuit boards.
Flexible printed circuits (hereinafter, abbreviated as “FPC”) which are thin and bendable are frequently used in the internal wiring of electronic equipment such as mobile telephone devices or digital cameras. In recent years, the use of FPCs has grown with the increasing compactification of portable devices and the rising number of moving parts. When jointing an FPC to a hard board which is used as a main circuit board, generally connectors are used, and these have a significant advantage in that they enable the FPC to be attached and detached repeatedly.
Even if attachment and detachment are not necessary, there is an advantage in that boards can be jointed easily. However, when jointing with connectors, the three-dimensional space occupied by the connectors is an obstacle to making equipment more compact and reducing its thickness. Furthermore, the minimum pitch of present-day connectors is generally 0.3 mm, and it is difficult to joint electrode terminals having a narrower pitch than this.
On the other hand, there are also rigid flex boards in which a hard board and an FPC are integrated completely. A rigid flex board has an advantage in that it does not require connections on the outer perimeter since an FPC is sandwiched in an inner layer of a hard board, but the manufacturing process is long and complicated steps are involved in assembling hard boards having different numbers of layers.
In these circumstances, recently, it has been possible to manufacture circuit boards having a similar structure to rigid flex boards, by jointing together different hard circuit boards by means of an FPC. In this way, it is possible to simplify the steps compared to those involved in a rigid flex board, and the outer shape and structure of the circuit board are less likely to be restricted.
Therefore, it could be considered effective to use prior art technology which self-assembles solder powder in order to joint boards which have electrode terminals arranged at narrow pitch of this kind.
On the other hand, the present inventors discovered phenomena of the following kind when the method described above is adapted to joint one circuit board to another circuit board. These phenomena are described below.
FIG. 12 shows a wiring board used to investigate jointing.
A band-shaped plurality of wires 33a are provided on a board 31a, and a connecting terminal (hereinafter, called connecting terminal 34a) is formed in the end region 34a of the board 31a. 
The wiring rules are as follows: the width of the wires 33a is 0.05 mm, a space 35a between mutually adjacent wires is 0.05 mm and the pitch is 0.1 mm.
An appropriate amount of resin 114 containing solder powder and a bubble generating agent (not illustrated) is applied to the central portion of the connecting terminal 34a of the board 31a. 
Next, as shown in FIG. 13A and FIG. 13B, the end portion of a separate board 31b to the board 31a is placed in overlapping fashion over the board 31a to which the resin 114 has been applied in FIG. 12, and the connecting terminal in an end region 34b of the board 31b (hereinafter, called “connecting terminal 34b”) is placed so as to oppose the connecting terminal 34a of the board 31a via the resin 114. Here, the connecting terminals 34a and 34b respectively have the same dimensions and the same pitch.
When the resin 114 is heated in this state, it is expected that the solder powder in the resin 114 self-assembles in the region where the connecting terminal 34a and the connecting terminal 34b overlap, and then melts and solidifies, thereby jointing the board 31a and the board 31b. 
However, when the heating step was actually carried out, a large amount of resin 114 and solder powder moved outside the region of overlap between the connecting terminal 34a and the connecting terminal 34b, as shown in FIG. 14. There was particularly marked movement of the resin 114 and the solder powder from the spaces 35a between the mutually adjacent wires on the board 31a and spaces 35b between mutually adjacent wires on the board 31b. 
When the overlapping portion between the board 31a and the board 31b was observed with an X-ray fluoroscope, the solder powder which had moved and assembled as shown in FIG. 15 had melted and solidified. Numeral 16a is a portion where the solvent powder has assembled and solidified outside the connection region, numeral 16b is a portion where there is insufficient solder in the connecting terminal, and numeral 16c is an unconnected portion. Not all of the solder powder had assembled into the overlapping region of the connecting terminal 34a and the connecting terminal 34b. 
In this way, it was seen that it is necessary to eliminate the problems described above, in order to joint boards having a connecting terminal arranged in a fine band shape by using self-assembly of conductive particles, such as solder powder, onto the electrodes.